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Self-assembled cactus-like NH 4 V 3 O 8 is prepared through polyethylene glycol (PEG)-assisted hydrothermal synthesis method. The samples are characterized by X-ray diffraction and scanning electron microscopy techniques. Adjusting the pH value of the solution can effectively control the morphology of NH 4 V 3 O 8. The electrochemical measurements indicate that the sample which pH value is 3.7, shows the best cycling performance. It delivers an initial discharge capacity is 228 mAh g−1 and maintains at 156 mAh g−1 under 15 mA g−1 after 50 cycles. The proposed synthetic method can also be developed to address other electrode materials with inferior electronic conductivity and lithium ion diffusion. Schematic illustration of cactus-like NH 4 V 3 O 8 by PEG4000-assisted hydrothermal method Image, graphical abstract [ABSTRACT FROM AUTHOR]

• A core–shell structure polypyrrole coated Cu/Cu 2 O nanowires was synthesized. • Li-storage performance of Cu 2 O can be enhanced by introduction of Cu metal and Ppy. • The high capacity of 401 mAh g−1 is still delivered at 1 Ag−1 after 500 cycles. • A phase transformation is proved to be the main reasons for capacity increase. A core-shell structure polypyrrole coated Cu/Cu 2 O (Cu/Cu 2 O@Ppy) nanowires was synthesized by a facile hydrothermal method. Benefiting from the nanowire morphology, the confinement effect of Ppy shell and the modification of Cu metal, the Cu/Cu 2 O@Ppy as the anode of lithium ion battery exhibits an increased reversible capacity of 787 mAh g−1 under the current density of 0.1 A g−1 after 250 cycles. In addition, a good long-life rate capacity also can be obtained (401 mAh g−1 at 1 A g−1 after 500 cycles). More to the point, the increased capacity of Cu/Cu 2 O@Ppy nanowires is explained by the analysis of capacity differential curve, ex-situ X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. It was demonstrated that the constant formation of solid electrolyte interface film and the transformation of Cu to Cu 2 O lead to the increased capacity in the initial cycles. Furthermore, a phase transformation of host material from Cu 2 O to CuO is proved to be the main reasons for the subsequent cycles of capacity increase. This in-situ phase transformation of electrode material provides a novel perspective for the increased capacity phenomenon of metal compounds in lithium ion battery. [ABSTRACT FROM AUTHOR]